Design and Control of Compliant Tensegrity Robots Through Simulation and Hardware Validation

Design and Control of Compliant Tensegrity Robots Through Simulation and Hardware Validation 1,2 1,3 1,4 1,5 KEN CALUWAERTS ,JÉRÉMIE DESPRAZ ,ATIL ISÇEN¸ ,ANDREW P. S ABELHAUS , 1,6 2 1,7 JONATHAN BRUCE ,BENJAMIN SCHRAUWEN , AND VYTAS SUNSPIRAL 1Dynamic Tensegrity Robotics Lab, NASA Ames Research Center, USA 2Reservoir Lab, Electronics and Information Systems department, Ghent University, Belgium [email protected], [email protected] 3Biorobotics Laboratory, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland jeremie.despraz@epfl.ch 4Oregon State University, USA [email protected] 5Berkeley Institute of Design, University of California Berkeley, USA [email protected] 6USRA/University of California Santa Cruz, USA [email protected] 7SGT Inc., NASA Ames Intelligent Robotics Group, USA [email protected] Abstract To better understand the role of tensegrity structures in biological systems and their application to robotics, the Dynamic Tensegrity Robotics Lab at NASA Ames Research Center has developed and validated two software environments for the analysis, simulation, and design of tensegrity robots. These tools, along with new control methodologies and the modular hardware components developed to validate them, are presented as a system for the design of actuated tensegrity structures. As evidenced from their appearance in many biological systems, tensegrity ("tensile-integrity") structures have unique physical properties that make them ideal for interaction with uncertain environments. Yet, these characteristics make design and control of bio-inspired tensegrity robots extremely challenging. This work presents the progress our tools have made in tackling the design and control challenges of spherical tensegrity structures. We focus on this shape since it lends itself to rolling locomotion. The results of our analyses include multiple novel control approaches for mobility and terrain interaction of spherical tensegrity structures that have been tested in simulation. A hardware prototype of a spherical six-bar tensegrity, the Reservoir Compliant Tensegrity Robot (ReCTeR), is used to empirically validate the accuracy of simulation. Keywords: tensegrity, bio-inspired locomotion, central pattern generators, reservoir computing, compliant robotics, soft robotics, planetary exploration This is a pre-copyedited, author-produced PDF of an article accepted for publication in Journal of the Royal Society Interface. The definitive publisher-authenticated version is available online at: http://dx.doi.org/10.1098/rsif.2014.0520 or http://rsif.royalsocietypublishing.org/content/11/98/20140520. Please cite this work as: Caluwaerts K, Despraz J, Iscen A, Sabelhaus AP, Bruce J, Schrauwen B, SunSpiral V. Design and control of compliant tensegrity robots through simulation and hardware validation. J. R. Soc. Interface September 6, 2014; doi:10.1098/rsif.2014.0520 1742-5662 1 Design and Control of Compliant Tensegrity Robots • K. Caluwaerts et al. • J. R. Soc. Interface I. Introduction have no rigid connections between them [6]. This new view is challenging the "common sense" view of skeletal structures as Prior work has investigated the unique structural properties of the primary load-bearing elements of human and mammalian tensegrity systems, their role in biology, and control strategies bodies (see Figure 2). In the emerging "bio-tensegrity" model, for different tensegrity morphologies. One of the centers for bones are still under compression, but they are not passing com- this research is NASA Ames Research Center, where there is pressive loads to each other; rather, it is the continuous tension interest in these systems for planetary exploration missions. network of fascia that are the primary load-bearers [5, 6]. I.1 Tensegrity Structures Tensegrity structures are composed of compression elements encompassed within a network of tensional elements; conse- quently, each element experiences either pure compression or pure tension. This allows individual elements to be extremely lightweight, as designs do not need to resist bending or shear forces. Active motion in tensegrity structures can be performed with minimal energy expenditure since actuators work linearly along load paths in tension elements, avoiding torques caused by long lever arms of traditional robotic designs. A unique property of tensegrity structures is how they inter- nally distribute forces. Since there are no lever arms, forces do not magnify around joints or other common points of failure. Rather, externally-applied forces distribute through the Figure 2: Tensegrity models of the spine show how vertebrae structure via multiple load paths, creating system-level me- float without touching. Image courtesy of Tom Flemons (copy- chanical robustness and tolerance to forces applied from any right 2006) [7] direction or failure of individual actuation elements [1]. Thus, tensegrity structures are ideally suited for operation in dynamic environments where contact forces cannot always be predicted. I.3 Tensegrity Robotics for Space Exploration NASA is supporting research into tensegrity robotics to create I.2 Tensegrity and Biology planetary rovers with many of the same qualities that bene- fit biological systems. The high strength-to-weight ratio of tensegrity structures is attractive due to the impact of mass on mission launch costs. Likewise, large tensegrity structures are deployable from compact configurations, enabling them to fit into space-constrained spacecraft. While these qualities have inspired studies of deployable antennae and other large Figure 1: Computer simulations of a nucleated tensegrity cell space structures [8], the unique force distribution of tensegrity model exhibits mechanical coupling between the cell, the cy- robots has only recently been investigated for planetary ex- toskeleton, and the nucleus. Adapted by permission from ploration [9]. Initial work in the NASA Innovative Advanced Macmillan Publishers Ltd: Nature Reviews Molecular Cell Concepts (NIAC) project [10] shows that controllable compli- Biology [2], 2009 ance and force distribution properties allow for reliable and robust environmental interactions during landing and planetary Tensegrity structures are being discovered in many aspects surface exploration. of biological systems, which motivates this work’s bio-inspired A key goal of this NASA work is to develop a tensegrity modeling and controls approaches. The tensegrity concept ap- probe with an actively controlled tensile network, enabling pears at various scales, from the cytoskeleton of individual cells compact stowage for launch followed by deployment for land- (see Figure 1) [2, 3, 4] to mammalian physiology [5]. Emerg- ing. Compliant tensegrity probes can safely absorb significant ing biomechanical theories are shifting focus from bone-centric impact forces, enabling high-speed Entry, Descent, and Land- models to fascia-centric models. Fascia is the connective tissue ing (EDL) scenarios where the probe acts like an airbag [9]. in our bodies (including muscles, ligaments, tendons, etc.) that However, unlike an airbag that must be discarded after a single forms a continuous web of tension, even surrounding and sup- use, the tensegrity also provides rolling mobility (Figure 3). porting bones, which, unlike traditional mechanical systems, This enables compact and lightweight planetary exploration 2 Design and Control of Compliant Tensegrity Robots • K. Caluwaerts et al. • J. R. Soc. Interface trol algorithms based on Central Pattern Generators (CPGs), distributed learning, and genetic algorithms instead of more traditional control approaches. I.4.1 Central Pattern Generators CPGs are neural circuits found in both vertebrate and inverte- brate animals that produce rhythmic patterns of neural activity without receiving rhythmic inputs. CPGs have been studied from a biological perspective and have been used extensively in robotics [21]. The term central indicates that rhythms of the CPG are not driven by sensory input, but are self-generated. CPGs are the fundamental building blocks for locomotor neural circuits in many animals, and are also key to other fundamental rhythmic activities such as chewing, breathing, and digesting. Recent research shows a close relationship between CPGs and motion primitives in the spine that enable both rhythmic and Figure 3: Mission Scenario - A tightly packed set of tensegri- discrete motions [22]. Alongside their biological inspiration ties expands, spreads out, falls to surface of moon, and then for use in robotic motion, CPGs present several interesting and safely bounces on impact. The same tensegrity structure cush- useful properties including distributed control, robustness to ioning landing is then used for exploration. perturbations, inherent tolerance to redundancies, fast control loops, and the ability to modulate locomotion by simple control missions with the capabilities of traditional wheeled rovers, signals. but with a mass and cost similar to a stationary probe. Dual CPGs are therefore well suited for controlling tensegri- use of structure allows a tensegrity mission to have a high ties [23] and other biomimetic structures [24]. Additionally, mass fraction between science payload and overall weight (as there is intuition for pairing tensegrity robots with CPG net- measured at atmospheric entry). This reduces mission cost works: the dynamics of physical forces propagating through and enables new forms of surface exploration utilizing the a tensegrity

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